Conjugated linoleic acid is a general term used to name positional and geometric isomers of linoleic acid. Conjugated linoleic acid (CLA) differs from ordinary linoleic acid in that ordinary linoleic acid has double bonds at carbon atoms 9 and 12. The common denominator of these conjugated acids is that there in only one single bond between the two double bonds. Examples of CLA include cis- and trans-isomers (E/Z isomers) of the following positional isomers: 2,4-octadecadienoic acid, 4,6-octadecadienoic acid, 6,8-octadecadienoic acid, 7,9-octadecadienoic acid, 8,10-octadecadienoic acid, 9,11-octadecadienoic acid, 10,12-octadecadienoic acid, and 11-13-octadecadienoic acid. The conjugated forms are considered more stable than the non-conjugated forms from a thermodynamic point of view.
While there are many possible cis-trans possible isomers, two forms are most abundant: trans,9-cis,11-octadecadienoic acid and trans,10-cis,12-octadecadienoic acid. In ruminant animals, certain bacteria in the rumen covert the linoleic acid of vegetable oils mainly to the trans,9-cis,11-octadecadienoic acid, which is why it is called rumenic acid.
In the areas of health and nutrition, researchers have shown that ingestion of conjugated fatty acids may inhibit tumor growth, prevent heart disease, and reduce body fat. There is a great deal of interest in the apparent health benefits imparted by certain conjugated linoleic acids. CLAs, originally isolated from the fat and milk of ruminants, exhibit impressive physiological effects in animal studies. In a variety of chemical forms, including but not limited to free fatty acids and fatty acid methyl esters, CLA reportedly has antidiabetic properties, leads to reduced carcinogenesis and atherosclerosis, and increases bone and muscle mass.
Conjugated linoleic acid has a significant potency relative to other fatty acids with respect to modulating tumorigenesis. As noted above, conjugated linoleic acid is closely related to linoleic acid but differs from linoleic acid in the position and configuration of the double bonds. Linoleic acid has a stimulatory effect on carcinogenesis, as contrasted with the ability of conjugated linoleic acid to inhibit tumor development. In this way, conjugated linoleic acid has the opposite effect of linoleic acid in treating carcinomas. In fact, conjugated linoleic acid has a significant potency relative to other fatty acids in modulating tumorigenesis.
The terms “conjugated linoleic acid” and “CLA” as used herein are intended to include 9,11-octadecadienoic acid, 10,12-octadecadienoic acid, and mixtures thereof. The non-toxic salts of the free acids may be made by reacting the free acids with a non-toxic base.
Conjugated linoleic acid has long been of interest to biochemists and nutritionists. An article in Inform, Vol. 7(2): 1996, published by the American Oil Chemists' Society, summarizes some of the data developed to that date. The article stresses the feed use for which the product was being developed, resulting in less fat and more lean meat in animals. A number of other recent articles stress the effects of CLA in fighting cancer. In many cases, one isomer, 9,11-CLA has been named as the active isomer, mainly because it alone is incorporated into the phospholipids of the organisms being fed CLA.
CLA has also been shown to have preventive effects on breast cancer in mice. CLA is not currently used for humans as a medication because it is only available in an impure form. Impurities in CLA can have a detrimental influence on toxicity tests required to obtain FDA approval.
Since CLA occurs naturally in foodstuff the FDA did not remove it from the market, but the FDA never approved the claims for its benefits.
According to Turner, Food Product Design, October, 2003, a 50/50 blend of CLA isomers impacts a person's body mass by reducing body fat while maintaining lean muscle mass when ingested at a recommended daily intake level of 3 grams.
The c9-11 isomer is associated with anticarcinogenic properties, possibly providing benefits in all three stages of caner, namely initiation, promotion, and metastasis.
The January, 2003 issue of Journal of Nutrition reports results of a human study conducted by Martha Belury, professor of human nutrition at Ohio State University, Columbus, that revealed the t-10,c-12 isomer impacted adult-onset (type 2) diabetes by lowering the subjects' body mass as well as blood sugar levels.
Gaullier et al., American Journal of clinical Nutrition 79:1, 118–125, 2004, reviewed various human health studies that investigated the effect of CLA on body composition. The research involved doses of CLA (50:50 mixtures of c-9,t-11 and t-10,C-12 isomers) ranging from 3 to 7 grams of CLA per day and treatment period running from four weeks to one year. Fat losses of up to 9% were reported, as well as 2–3% increases in lean mass.
Further benefits of CLA include the following:
                Increases metabolic rate        Decreases abdominal fat        Enhances muscle growth        Lowers cholesterol and triglycerides        Reduces food-induced allergic reactions        Enhances the immune system        
A factor hampering commercialization and research interest in CLA is that these compounds are not naturally abundant. Conjugated polyenes are typically present in animal fats only at a level of about 0.5 percent. Conjugated polyenes do not occur widely in plants.
Several methods exist for preparing conjugated fatty acids, including biosynthesis, dehydration of hydroxy fatty acids, and isomerization. Biosynthetic methods have been used to prepare a number of conjugated dienes. This technique was the results of a discovery that bacteria found in the stomachs of ruminants convert dietary unsaturated fatty acids contained in plant food sources into conjugated isomers. For example, the enzyme linoleate isomerase, isolated from the rumen anaerobic bacterium Butyrivibrio fibrisolvens, isomerized linoleic acid to main cis-9, trans-11-octadecadienoic acid, or rumenic acid. However, biosynthetic methods are not preferred for several reasons, including generally low yields and the difficulty of isolating specific conjugated compounds from the mixture that results.
In preparing conjugated fatty acids via dehydration of hydroxy fatty acids, various isomers can be obtained. However, although these methods produce yields that are somewhat better than biosynthetic methods, yields seen in such dehydrogenation methods nevertheless are still less than about 70%.
Synthesis of conjugated fatty acids via isomerization typically proceeds from an unconjugated polyene fatty acid or fatty acid ester as precursor, particularly linoleic acid. According to the delta nomenclature system, linoleic acid can be expressed as all-cis-9,12-octadecadienoic acid or c-9,c-12-octadecadienoic acid.
Isomerization produces various isomers that have the same atomic composition as the parent compound but that differ in chemical structure. Isomerization of linoleic acid could produce a total of at least eight isomers: two positional isomers, α9,11 and α10,12. Each of the positional isomers could appear as four geometric isomers: c-9,c-11; c-9,t-11; t-9, t-11- and t-9,c-11; and c-10,c-12-, c-10,t-12-, t-10,c-12, and t-10,t-12.
Isomerization of unconjugated polyenes to produce conjugated polyenes can be accomplished in several ways, including photochemically, by means of metallic ion or metal carbonyl catalysts, treatment with acids, and treatment with strong bases.
A typical photosensitization process involves irradiating an unconjugated precursor with light of a suitable wavelength range in a solvent and optionally in the presence of a suitable photosensitizer. Disadvantages of this method include the need for special equipment and the need to remove residual photosensitizer from the final product. Moreover, yields obtained typically are only about 80%.
Double-bond migrations can also take place by means of treatment with metallic ions (most often, complexes containing Pd, Pt, Rh or Ru) or metal carbonyl catalysts. This type of isomerization proceeds according to one of two possible mechanisms. The first mechanism, known as the metal-hydride addition-elimination reaction, requires external hydrogen. The second method, called the α-allyl complex mechanisms, does not require external hydrogen. In either case, however, the transition metals typically required in this type of isomerization are expensive and sometimes toxic.
Double-bond rearrangements can also take place upon treatment with acids. This type of isomerization follows a two-step mechanism, in which one of the double-bonded carbon atoms first gains a proton, giving a carbocation, and then the methylene unit adjacent to the other doubly-bonded carbon atom loses a proton, causing a double bond to re-form. The most thermodynamically stable isomer is the one predominantly formed during isomerization. Acid-catalyzed isomerization is not a preferred method, however, because carbocations generate many side products in addition to the desired conjugated isomers.
The most common isomerization methods used to produce conjugated fatty acids involve treatment of unsaturated fatty acids with strong base. As in the case of the acid-catalyzed isomerization reaction, base-catalyzed isomerization produces equilibrium mixtures of the most thermodynamically stable isomers. Base-catalyzed double bond isomerization, sometimes called prototropic rearrangement, is an example of electrophilic substitution with accompanying allylic rearrangement. Because the double bond of the unconjugated substrate can shift to be in conjugation with the one already present, the double bond will migrate that way because the conjugated configuration is more thermodynamically stable.
Various methods can be used to produced conjugated fatty acids by base-catalyzed isomerization of an unconjugated fatty acid, but all of these methods have certain drawback. For example, A.O.C.S. Official Method Cd 7-58, produced conjugated compounds by adding a solution of potassium hydroxide in ethylene glycol to an unconjugated substrate in a weight ratio of about 110:1 and maintaining the reaction at 180° C. for 25 minutes. This process uses a considerable excess of alkali metal hydroxide catalyst, adding expense and presenting safety concerns.
Another problem with many CLA products made by conventional approaches is their heterogeneity, and substantial variation from batch to batch. Commercial preparations contain between 50 and 75% of CLA. Commercial CLA is generally made from safflower oil, which oil contains a substantial amount of linoleic acid. However, the reaction used for conversion of cis-linoleic acid to CLA takes place at very high temperatures (200–250° C.). This high temperature converts the oleic acid to 50% trans-oleic acid, a compound known to be atherogenic and carcinogenic. Other processes for preparing CLA produce essentially a combination of about 48% trans-9,cis-11 and 47% trans-10,cis-12 conjugated linoleic acids, and the balance consists of other isomers of unknown biological activity. Therefore, there exists a great need for biologically active CLA products of defined composition.